Conventional phase masks have been developed over the past several decades to produce a controlled phase profile for an optical system. They have been recorded using a wide variety of substrates such as photoresist and dichromated gelatin but in each case the principle behind the element is the same. In order to create the local phase profile the local optical path length is controlled, whether by controlling the geometrical path length or the local refractive index. In either case, because the optical path length is controlled, the phase profile is designed for a specific wavelength corresponding to that optical path length difference. This inherently limits conventional phase masks to uses in monochromatic systems as in broadband systems it is impossible to provide the same optical path length difference for every wavelength.
The use of holographic phase masks has been demonstrated in the literature for thin films where the probe wavelength is the same as the recording wavelength. See, Y. Ishii and T. Kubota, “Wavelength demultiplexer in multimode fiber that uses optimized holographic optical elements,” Applied Optics 32, 4415-4422 (1993); Aoki et al., “Selective multimode excitation using volume holographic mode multiplexer,” Optics Letters 38, 769-771 (2013); D. Flamm et al., “All-digital holographic tool for mode excitation and analysis in optical fibers,” Journal of Lightwave Technology 31, 1023-1032 (2013); and Akayam et al., “Mode demultiplexer using angularly multiplexed volume holograms,” Optics Express 21, 012920 (2013), all of which are hereby incorporated in their entireties by reference.
The recording medium for VBGs is photo-thermo-refractive (PTR) glass, which is a photosensitive glass with a high damage threshold, low absorption, and wide transparency region, making it a suitable substrate for high power systems [See, L. B. Glebov, “Photochromic and photo-thermo-refractive (PTR) glasses,” Encyclopedia of Smart Materials, John Wiley & Sons, NY, 770-780 (2002); Oleg M. Efimov, Leonid B. Glebov, Larissa N. Glebova, Vadim I. Smirnov. Process for production of high efficiency volume diffractive elements in photo-thermo-refractive glass. U.S. Pat. No. 6,586,141 B1. Jul. 1, 2003; and Oleg M. Efimov, Leonid B. Glebov, Vadim I. Smirnov. High efficiency volume diffractive elements in photo-thermo-refractive glass. U.S. Pat. No. 6,673,497 B2. Jan. 6, 2004, all of which are hereby incorporated in their entireties by reference.
This glass is photosensitive in near UV spectral region and it is transparent from 350 to 2700 nm. This medium was successfully used for volume phase masks [See, Marc SeGall, Vasile Rotar, Julien Lumeau, Sergiy Mokhov, Boris Zeldovich, and Leonid B. Glebov. Binary volume phase masks in photo-thermo-refractive glass. Opt. Lett. 37 (2012) 190-192, the entirety of which is hereby incorporated by reference.] was found for this glass and was used for recording of both diffracting and refractive optical elements [See, L. B. Glebov and V. I. Smirnov. Interaction of photo-thermo-refractive glass with nanosecond pulses at 532 nm. Laser-Induced Damage in Optical Materials. Ed. G. J. Exarhos, A. H. Guenther, N. Kaiser, K. L. Lewis, M. J. Soileau, C. J. Stolz. Proceedings of SPIE 5273 (2004) 396-401; Leo Siiman, Julien Lumeau, Larissa Glebova, Vadim Smirnov, Leonid B. Glebov. Production of high efficiency diffractive and refractive optical elements in multicomponent glass by nonlinear photo-ionization followed by thermal development. U.S. Pat. No. 8,399,155; Mar. 19, 2013, all of which are hereby incorporated in their entireties by reference.]
Past phase masks achromatization concepts have involved several different techniques involving additional phase masks, birefringence, and thin films [See, R. Galicher, P. Baudoz, and J. Baudrand. Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes. A&A 530, A43. ESO. Mar. 28, 2011; D. Mawet, P. Riaud, J. Baudrand, P. Baudoz, A. Boccaletti, O. Dupuis, and D. Rouan. The four-quadrant phase mask coronagraph: white light laboratory results with an achromatic device. A&A 448, 801-808. ESO. Nov. 8, 2006; P. Riaud, A. Boccaletti, D. Rouan, F. Lemarquis, and A. Labeyrie. The four-quadrant phase-mask coronagraph. ii. simulations. Astronomical Society of the Pacific, Vol. 113, No. 787. pp. 1145-1154. September 2001, each of which is hereby incorporated in their entirety by reference]. Multiple phase masks have been used together in attempt to minimize the presence of other wavelengths. Individual phase masks are designed for a specific wavelength and then placed subsequent to one another. A second concept is the use of birefringence in materials as half wave plates, and create the same effect as a phase mask with the resulting polarization changes. Layers of thin films have also been proposed based on phase differences resulting from reflections.
Volume Bragg gratings (VBGs) are diffractive optical elements fabricated in a transparent optical material which possess periodical variation of refractive index in one direction. A VBG provides diffraction of an incident optical beam if it has a proper wavelength and launched at a proper angle of incidence (Bragg condition). An ideal VBG has a uniform average refractive index and a uniform spatial refractive index modulation. These features enable fine spectral and angular selection when diffracted beams have no induced phase distortions. Such VBGs are recorded in photosensitive media by exposing them to an interference pattern produced by coherent collimated beams with uniform spatial distribution of intensity and phase. One important peculiarity of VBGs is the ability to multiplex multiple elements in the same volume of a photosensitive medium. It enables creation several optically independent elements in the same volume.
Phase masks are optical elements which provide different optical path lengths across an aperture. A spatial phase profile (spatial profile of optical path which is a product of refractive index and geometrical thickness) is produced by shaping of surface of corresponding optical elements (conventional surface phase masks) or by spatial variations of refractive index (volume phase masks). The general feature of all phase masks is their ability to transform modes of light propagation. It is clear that phase masks work at only specified wavelength because the phase shift is uniquely determined by a product of refractive index and thickness.
A new recently invented type of phase masks (Leonid Glebov, Ivan Divliansky, Marc SeGall. Holographic phase masks recorded in volume Bragg gratings. U.S. Non-Provisional patent application filed on Oct. 23, 2014 as Ser. No. 14/521,852, the entirety of which is hereby incorporated by reference) is fabricated by the interference of coherent beams with specific phase profiles. Such phase masks, also called holographic phase plates (HPLs). These complex optical elements provide diffraction of an incident beam (as conventional VBG) if the angle of incidence corresponds to the Bragg angle for a given wavelength. However, different parts of the diffracted beam have specific mutual phase relations determined by phase relations in the recording beam. The HPL will operate only when it is illuminated with a specific wavelength at the volume grating's Bragg angle. This means that HPL can be used at different wavelengths if it is angularly tuned in order to meet the corresponding Bragg condition. It is well known that holograms in general possess high chromatism and can be reconstructed only at the same wavelength that was used for recording. However, it is an inherent property of uniform VBGs that by proper choice of incident angle, diffraction can be obtained for different wavelengths. This effect is provided by changing incident angles to satisfy Bragg condition for different wavelengths. This VBG inclination automatically provides changing of phase incursion for a propagating beam and, therefore, keeps the phase profile in the diffracted beam constant for any wavelength (if phase shift is measured in wavelengths). This is why, contrary to conventional phase masks, holographic phase masks imbedded in VBGs are tunable and can operate at any wavelength that can satisfy Bragg condition for a recorded VBG.
It is an object and advantage of the present invention to provide near-diffraction-limited high-power beams with wide spectra by taking advantage of the high power capacity of large-mode-area fibers which generate undesirable higher order modes and then converting them to the fundamental mode and combining them into a single high-power beam.
It is another object and advantage of the present invention to employ as a multiplexer/demultiplexer.
It is a further object and advantage of the present invention to provide a system that is easily manufactured without the need of expensive precision thickness measurements or birefringent crystal structures.
Other objects and advantages of the present invention will in part be obvious and in part appear hereinafter.